He was introduced in the sixth episode of the fourth season, "Oh Brother, Where Art Thou?". Quinn Mallory had only learned Colin was his brother five episodes earlier in "Genesis". When the sliders found Colin, he was flying a hangglider that he invented, and had gotten stuck in a tree. The local residents were angry at Colin for doing this because they were afraid his inventions might have negative consequences. One of the residents wanted to shoot Colin to get it over with, until the sheriff told the angry residents to go home. When the sheriff said, "One of these days you're going to go a bit too far, Colin Mallory!" the sliders knew that this was Quinn's brother.
The universe Colin was raised on had far less technology than Earth Prime. Colin invented the first machine on this earth that generates electricity. This was one of his many inventions. The local residents believed that Colin was "doing the devil's work" with his inventions. Colin believed that electricity is both safe and useful and wished he could convince the local residents the same thing. Colin was in love with someone named Suzanne on his earth. However, her mother disapproved Colin, because of her fear of Colin's inventions. Suzanne was engaged to another person. Colin's adoptive parents (duplicates on the world Colin was raised of Colin's actual parents) died when Colin was very young; since this earth had little technology, there was not the medical technology necessary to cure Colin's parents of their illness.
Since the sliders had a small amount of time on this world, they had to hurry to find the microdot that would inform Colin of his origins. They found it just in time, and Colin was informed that he was originally from Kromagg Prime and that he was placed on an alternate earth when he was young so that he could be safe. Quinn and Colin's birthparents presumably did look for Colin when the war with the Kromaggs was over, but Colin's adoptive parents were dead, so his birthparents were unable to find him.
After Colin saw the microdot, Colin was resistant to sliding with Quinn and the others, since Colin didn't want to leave his world and the woman he wanted to marry. Quinn did convince Colin that his future was sliding with him, so Colin went into the vortex.
Since Colin was raised on an earth with less technology than most earths the sliders visit, he was unfamiliar with the technology he encountered. On his first slide, Colin landed in the path of a moving vehicle. Being unaware that it would be lethal to be hit by the vehicle, Colin did not get out of the way as the vehicle approached him. Quinn had to move Colin out of the way.
In the episode "Revelations", Colin met who he thought were his birthparents but it turned out they were doubles.
In the episode "The Unstuck Man", Colin was made "unstuck" in the multiverse, meaning, that he was not anchored on a particular world. This was the result of an experiment by Dr. Oberon Geiger.
According to Marc Scott Zicree, a writer for the show, the retcon from "Genesis" was originally intended to be a red herring. The idea was that Colin was actually a clone of Quinn created by the Kromaggs as a sleeper agent within the Sliders. In the original draft for "Revelations," he would flip, attacking Quinn and ending the season on a cliffhanger.
This would probably have had the effect of simplifying the somewhat complicated (and largely financially unresolvable) plot progression of the show. Nevertheless, because this would render Charlie O'Connell's character unusable in a regular capacity after Season 4, and because by that time Jerry O'Connell's residency on the show depended on his brother's presence (and because this was all before anyone knew the brothers would be departing) David Peckinpah vetoed the idea. The veto of Zicree's idea was finalized with "Lipschitz Live," in which Colin has a double (which would mean he couldn't be a clone of Quinn, unless the double is also a clone).